Power converters are used in a wide variety of applications to provide power source to power electronics, charge batteries, and other applications. A typical converter converts a high level direct current (DC) voltage level down to a useable level. Some converters include an alternating current (AC) to DC conversion at the front end of the converter to convert standard AC power service (e.g. 120 VAC) to the high DC, which is then converted down (or up) to the desired level. Some converters are used to provide a stable voltage level, a stable current level, or alternately either a stable voltage or current level, as in the case of many battery chargers.
Conventional power converters are switch mode converters which utilize a converter inductance to regulate the conversion, in conjunction with a switching device to load the conversion inductance and then switch to an output, which is then capacitively filtered to stabilize the output. The conversion inductance can be, for example, a winding on a transformer, or a simple inductor. Examples of switched mode converters that are well known include buck, boost, buck/boost, flyback, push-pull, half bridge, and full bridge converters.
Generally, power converters use a switch transistor connected in series with a conversion inductance to draw current through the conversion inductance by closing the switch transistor (i.e. putting the switch transistor in a saturation state), resulting in energy being stored in a magnetic field of the conversion inductance in response to the current. When the switch transistor is opened (i.e. put into a high impedance state), the energy in the magnetic field is dispersed to the regulated side of the converter where a bulk filter capacitance is generally used to smooth out the energy transfers to an acceptable level. To control the amount of energy being transferred by the conversion inductance the switch transistor is commonly switched using pulse width modulation (PWM) where a substantially squared pulse signal is switched at a particular frequency, and the duty cycle of the pulse width is varied in correspondence with the power demand on the regulated side of the converter.
Since a transistor does not switch instantly from a high impedance state to a low or saturated state, losses occur in the transition between those two states (i.e. through the active and linear regions). Accordingly, the conventional approach to minimizing losses, and to maximize converter efficiency, is to switch as fast as possible. Switching speeds are dependent on the output characteristics of the PWM signal and the switch transistor. As a result of switching the switch transistor as quickly as possible, transients result across the conversion inductance. These transients typically require suppression to comply with governmental conducted and radiated emissions standards. However, the transients often need to be suppressed even more than that required by governmental standards in order to avoid interference with nearby circuitry and other systems. Suppression of transients resulting from switching is typically accomplished by connecting filtering and dissipating components, such as capacitors, resistors, and non-linear steering devices (e.g. diodes) across the conversion inductance. While these components can effectively suppress transients to achieve a desired performance specification regarding conducted and radiated emissions, it is also a source of inefficiency as the power of the transients are being dissipated as heat. In addition to reducing efficiency, the transient handling components add to the manufacturing cost of the converter.
Accordingly, there is a need for a power converter circuit that has reduced transients so that transients do not have to be dissipated and to avoid interfering with nearby devices through conducted or radiated emissions resulting from transients.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.